South Pole detector spots 28 out-of-this-world neutrinos

Extremely high energy particles originated from outside the Solar System.

Earlier this year, scientists using a powerful detector at the South Pole discovered Ernie and Bert, two neutrinos with energies over 100 times higher than the protons that circulate in the LHC. Now, the same team has combed through its data to find an additional 26 high-energy events, and they've done a careful analysis to show that these are almost certainly originating from somewhere outside our Solar System.

Neutrinos are incredibly light particles that rarely interact with normal matter; staggering numbers pass through the Earth (and your body) every second. To spot one, you need a very large detector, and IceCube fits the bill. Located in the ice cap at the South Pole, the detector works by capturing the light produced when neutrinos interact with the huge volume of ice present. To do so, holes were drilled up to 2 km into the ice, and strings of photodetectors were lowered into them. All told, they pick up the signals from a cubic kilometer of ice.

Further Reading

You know it's cold when you have to heat the air used to cool your data center.

The challenge is figuring out which signals come from the out-of-this-world neutrinos. Cosmic rays slam into the atmosphere all the time, and these can produce neutrinos that then enter the ice cap. They can also produce other exotic particles that produce light as they pass through the ice. Muons, for example, only live about 10-6 seconds, but they're moving so fast that time dilation means they live longer from the Earth's frame of reference. As a result, they may travel several kilometers through the ice before decaying.

To handle these cases of background, the authors eliminated any signals that were present in the outermost edges of the detector. Cosmic rays are especially easy to spot given that they tend to produce a spray of particles, many of which will be found at the detectors closest to the surface. You might still get a few neutrinos created above the North Pole and passing through on their way out of Earth, but the authors found that the majority of their signals came from the south, suggesting that these neutrinos aren't a major problem for this detector.

Previously, the authors' analysis only picked up very high-energy events; Ernie and Bert were about one Peta-electronVolt each (for comparison, the LHC's protons are at 4 Tera-eV). Now, they've extended the sensitivity down to as low as 30TeV, with 28 events spread throughout the range of energies between the two. Seven of these produced muons in the detectors, indicating that they were produced by the muon neutrino. The rest produced a shower of signals, suggesting that they originated from some other form of neutrino.

The energies and properties involved in these neutrinos indicate that they originated outside our Solar System. Just as cosmic rays can produce neutrinos when they slam into something nearby, energetic events can produce neutrinos that travel significant distances across the Universe. One example might be if the jets of particles from a black hole slammed into a gas cloud, producing unstable particles like pions that decay in ways that produce neutrinos. Since all that energy ends up in a particle that's only a billionth of the mass of a proton, the neutrinos end up effectively traveling at the speed of light. Meaning that if we can see something anywhere in the Universe, we can also detect any neutrinos it produces.

The downside is that we don't yet have the ability to work backward to figure out the direction that the neutrinos originated from. We can give a rough area of the sky, but it's not good enough to direct observatories to image the source. At least within the IceCube detector, there was also no apparent pattern in time, indicating that it wasn't able to pick up any burst events. Although we're pretty sure these came from outside our Solar System, we can't currently say much about what produced them.

The downside is that we don't yet have the ability to work backward to figure out the direction that the neutrinos originated from. We can give a rough area of the sky, but it's not good enough to direct observatories to image the source. At least within the IceCube detector, there was also no apparent pattern in time, indicating that it wasn't able to pick up any burst events. Although we're pretty sure these came from outside our Solar System, we can't currently say much about what produced them.

So basically this was the cosmic equivalent of The Most Interesting Man In The World knocking on your door and then running away, never to be seen again. Grrr...!

I recall a quote but can't find the reference now. Somebody help me out. Fermi maybe? I also may have the verbage slightly wrong. "I've done a terrible thing, predicting a particle which can't be detected"

I recall a quote but can't find the reference now. Somebody help me out. Fermi maybe? I also may have the verbage slightly wrong. "I've done a terrible thing, predicting a particle which can't be detected"

How bright is the light that the photodetectors are looking for? I'm got this notion of unbelievably dim flashes in my mind...

The human eye couldnt see it, but it's just as bright as any other photon of that wavelength (E=hf). So the detectors just need to be able to detect individual photons, but its hard to put a value on brightness. Incedentally, if the human eye had developed a to be more sensitive we would see this ourselves as dim light would appear as a series of equally bright flashes of light.

I recall a quote but can't find the reference now. Somebody help me out. Fermi maybe? I also may have the verbage slightly wrong. "I've done a terrible thing, predicting a particle which can't be detected"

How bright is the light that the photodetectors are looking for? I'm got this notion of unbelievably dim flashes in my mind...

The human eye couldnt see it, but it's just as bright as any other photon of that wavelength (E=hf). So the detectors just need to be able to detect individual photons, but its hard to put a value on brightness. Incedentally, if the human eye had developed a to be more sensitive we would see this ourselves as dim light would appear as a series of equally bright flashes of light.

From what I remember from the Sudbury science center (where they have SNO - the Sudbury Neutrino Observatory), trying to see a single photo is about the equivalent of trying to see a candle on the moon. The sensors in SNO were capable of picking up single photos. Pretty crazy stuff, when you think about it.

I recall a quote but can't find the reference now. Somebody help me out. Fermi maybe? I also may have the verbage slightly wrong. "I've done a terrible thing, predicting a particle which can't be detected"

How bright is the light that the photodetectors are looking for? I'm got this notion of unbelievably dim flashes in my mind...

The human eye couldnt see it, but it's just as bright as any other photon of that wavelength (E=hf). So the detectors just need to be able to detect individual photons, but its hard to put a value on brightness. Incedentally, if the human eye had developed a to be more sensitive we would see this ourselves as dim light would appear as a series of equally bright flashes of light.

From what I remember from the Sudbury science center (where they have SNO - the Sudbury Neutrino Observatory), trying to see a single photo is about the equivalent of trying to see a candle on the moon. The sensors in SNO were capable of picking up single photos. Pretty crazy stuff, when you think about it.

Off-the-shelf photomultiplier tubes are used in single photon counting mode all the time.

I recall a quote but can't find the reference now. Somebody help me out. Fermi maybe? I also may have the verbage slightly wrong. "I've done a terrible thing, predicting a particle which can't be detected"

Reminds me a bit on Babylon 5. The human tells one of the aliens he's just swallowed a microscopic tracking/listening device. After the alien leaves in a huff, his second-in-command asks why bother, they'd just find it and remove it.

"That's right. If there was a bug, they'd be able to find it.""Do you have any idea what they'll do to him looking for that thing?""Yes." <smile>

How bright is the light that the photodetectors are looking for? I'm got this notion of unbelievably dim flashes in my mind...

The human eye couldnt see it, but it's just as bright as any other photon of that wavelength (E=hf). So the detectors just need to be able to detect individual photons, but its hard to put a value on brightness. Incedentally, if the human eye had developed a to be more sensitive we would see this ourselves as dim light would appear as a series of equally bright flashes of light.

From what I remember from the Sudbury science center (where they have SNO - the Sudbury Neutrino Observatory), trying to see a single photo is about the equivalent of trying to see a candle on the moon. The sensors in SNO were capable of picking up single photos. Pretty crazy stuff, when you think about it.

I'm know I'm being an ass, but wouldn't a penlight be a more appropriate item than a candle on the moon?

How bright is the light that the photodetectors are looking for? I'm got this notion of unbelievably dim flashes in my mind...

The human eye couldnt see it, but it's just as bright as any other photon of that wavelength (E=hf). So the detectors just need to be able to detect individual photons, but its hard to put a value on brightness. Incedentally, if the human eye had developed a to be more sensitive we would see this ourselves as dim light would appear as a series of equally bright flashes of light.

From what I remember from the Sudbury science center (where they have SNO - the Sudbury Neutrino Observatory), trying to see a single photo is about the equivalent of trying to see a candle on the moon. The sensors in SNO were capable of picking up single photos. Pretty crazy stuff, when you think about it.

How bright is the light that the photodetectors are looking for? I'm got this notion of unbelievably dim flashes in my mind...

The human eye couldnt see it, but it's just as bright as any other photon of that wavelength (E=hf). So the detectors just need to be able to detect individual photons, but its hard to put a value on brightness. Incedentally, if the human eye had developed a to be more sensitive we would see this ourselves as dim light would appear as a series of equally bright flashes of light.

From what I remember from the Sudbury science center (where they have SNO - the Sudbury Neutrino Observatory), trying to see a single photo is about the equivalent of trying to see a candle on the moon. The sensors in SNO were capable of picking up single photos. Pretty crazy stuff, when you think about it.

I'm know I'm being an ass, but wouldn't a penlight be a more appropriate item than a candle on the moon?

Excellent point. You could of course see a candle on the moon. If it was large enough, or you were close enough. CandleLIGHT would be a different issue though;)

From the article: "...with 28 events spread throughout the range of energies between the two."Over what timeframe?

As long as the detector was running. There's no temporal relationship between the events.

Seriously? 28 events throughout its entire operation up to now? According to Wikipedia IceCube's been running at some capacity from 2005.

I have to wonder, what kind of models are they able to confirm or construct based on such a small data set? And as the article mentions, they can't infer the direction the neutrinos came from either. The only thing I can think of is guess-timating the frequency of interstellar events that produce high energy neutrinos.

Seriously? 28 events throughout its entire operation up to now? According to Wikipedia IceCube's been running at some capacity from 2005.

28 events of this high-energy type, that they believe are extra-solar, yeah. I'm sure they've seen a lot more neutrino events than just that.

These things are really hard to detect. SN1987a, a supernova (which produces a truly ridiculous number of neutrinos) in the Large Magellanic Cloud so just outside our galaxy, produced a total of 24 events across all the neutrino experiments running at the time.

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I have to wonder, what kind of models are they able to confirm or construct based on such a small data set? And as the article mentions, they can't infer the direction the neutrinos came from either. The only thing I can think of is guess-timating the frequency of interstellar events that produce high energy neutrinos.

It is unlikely to confirm any theory to any great precision, but it could easily rule out theories. Small sample sizes means big error bars, so if your theory falls within them it's hard to say "Ha! I'm right!" but if your theory falls outside them it's easy to say "Oh no, I'm wrong!"

From the article: "...with 28 events spread throughout the range of energies between the two."Over what timeframe?

As long as the detector was running. There's no temporal relationship between the events.

Seriously? 28 events throughout its entire operation up to now? According to Wikipedia IceCube's been running at some capacity from 2005.

I have to wonder, what kind of models are they able to confirm or construct based on such a small data set? And as the article mentions, they can't infer the direction the neutrinos came from either. The only thing I can think of is guess-timating the frequency of interstellar events that produce high energy neutrinos.

Regarding the next comment about Supernova 1987A, it's likely that such an event would produce about 100,000 detections at Ice Cube. Source: some other article I read this morning.

If you're not in a supernova-rich neighborhood or hanging out near pulsars or black holes, high-energy neutrinos are harder to come by.

I don't understand this part:"Muons, for example, only live about 10-6 seconds, but they're moving so fast that time dilation means they live longer from the Earth's frame of reference. As a result, they may travel several kilometers through the ice before decaying."

The frame of reference shouldn't have any bearing on how many km of ice they make it through before decaying, only how long it's observed to take, correct? Observing from any frame of reference, whether the earthbound observer, the particle, or some distant third point, the penetration would be the same wouldn't it?

I don't understand this part:"Muons, for example, only live about 10-6 seconds, but they're moving so fast that time dilation means they live longer from the Earth's frame of reference. As a result, they may travel several kilometers through the ice before decaying."

The frame of reference shouldn't have any bearing on how many km of ice they make it through before decaying, only how long it's observed to take, correct? Observing from any frame of reference, whether the earthbound observer, the particle, or some distant third point, the penetration would be the same wouldn't it?

Length contraction. In its own rest frame the muon lasts 10^-6 s, but it is moving so fast relative to Earth that the length from the upper atmosphere (or whatever) to the ice is much smaller than in our reference frame. We, on the other hand, observe the muon passing through the entire atmosphere to the surface as well, but in our frame of reference the lifetime of the muon before it decays is subject to time dilation.

Any theories why most of the neutrinos would be passing through from the south end? Are most of the events that produce them that way or are the others being pushed away from the detector (or absorbed) while traveling through the earth?

From the article: "...with 28 events spread throughout the range of energies between the two."Over what timeframe?

As long as the detector was running. There's no temporal relationship between the events.

Seriously? 28 events throughout its entire operation up to now? According to Wikipedia IceCube's been running at some capacity from 2005.

this analysis was only using data from two years, when the detector was (almost) complete. Since the analysis was looking for high energy events fully contained in the detector, you need the full detector volume (the addiations in the last year when IceCube was finished were only additional modules in the center of the detector).

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Any theories why most of the neutrinos would be passing through from the south end? Are most of the events that produce them that way or are the others being pushed away from the detector (or absorbed) while traveling through the earth?

At these high energies, the Earth is getting more and more opaque for neutrinos, so they get absorbed and you would expect to see more events on the southern sky.

From the article: "...with 28 events spread throughout the range of energies between the two."Over what timeframe?

It would be an interesting math problem to postulate that a group of these high energy neutrinos came from the same event and then solve for the distance and neutrino rest mass it would take for that to be possible.

Length contraction. In its own rest frame the muon lasts 10^-6 s, but it is moving so fast relative to Earth that the length from the upper atmosphere (or whatever) to the ice is much smaller than in our reference frame.

In the muon's rest frame it's earth that is moving. Not that there's anything remotely wrong with referring to it as you did (they're equivalent). I'm just trying to conjur up the image of a poor muon seeing the earth flying at it at nearly the speed of light.

Length contraction. In its own rest frame the muon lasts 10^-6 s, but it is moving so fast relative to Earth that the length from the upper atmosphere (or whatever) to the ice is much smaller than in our reference frame.

In the muon's rest frame it's earth that is moving. Not that there's anything remotely wrong with referring to it as you did (they're equivalent). I'm just trying to conjur up the image of a poor muon seeing the earth flying at it at nearly the speed of light.

A fair point, I was speaking a little loosely.

As for the muon, I believe the proper anthropomorphic reaction is "Ohcrapohcrapohcrapohcrap..."

First of all, in any kind of particle physics experiment, you never find the *actual* particle, but always some byproduct it produces. Even if it's something as normal as an electron, what you actually see is when it's detected in a szintillator is light produced by electrons in the szintillator when they return to their non-excited state.

Here they describe their method in the abstract. They looked for events (tracks or light cascades) that were started in the inner detector. If you see light suddenly produced in the detector, there must be something that travelled until there without producing light in the outer parts of the detector. So it must be electrically neutral. It must also be weakly interacting, because otherwise it wouldn't get that far (which eliminates neutrons and all other neutral particles we know). And that leaves us neutrinos.

It's important to know that these are not the first neutrinos ever found. They are just the ones with the highest energies. So there's a lot of experience how to detect and identify neutrinos, plus the measurements of low energy neutrinos fit to what's expected from measurements of cosmic rays (which produce, amongst others, neutrinos in our atmoshpere). So you can be reasonably sure that it's correct.

Edit: the Cerenkov radiation is produced by the particles created by the neutrinos, ot the neutrinos themselves, since only charged particles create Cerenkov radiation.